Propylene polymers, both homopolymers and copolymers, are well known in the art, and their methods of preparation and uses are many and varied. Depending upon, among other things, their compositional content (e.g., the nature and quantity of the monomer(s) from which the polymer is made), structure (e.g., the manner in which the units derived from the monomers arrange themselves into or onto a polymer chain), etc., the propylene polymer will have desirable or undesirable properties for a given application. One property, actually a combination of properties, important to processors of propylene polymers is processability, e.g., the shear thinning behavior and melt strength of the polymer in a melted state.
One class of propylene copolymers is that described in U.S. Pat. No. 6,525,157. These ethylene-propylene copolymers, when produced in the presence of a metallocene catalyst and an activator, in a single steady state reactor, show a balance of flexural modulus, tensile strength and elasticity. These copolymers are substantially free of diene-derived units, and exhibit softness, tensile strength and elasticity.
Another, more interesting class of propylene copolymers that exhibit a wide range of desirable properties is that described in U.S. Pat. No. 6,960,635. These polymers are characterized as comprising at least about 60 weight percent (wt %) of units derived from propylene, about 0.1-35 wt % of units derived from ethylene, and 0 to about 35 wt % of units derived from one or more unsaturated comonomers, with the proviso that the combined weight percent of units derived from ethylene and the unsaturated comonomer does not exceed about 40. These copolymers are also characterized as having at least one of the following properties: (i) 13C NMR peaks corresponding to a regio-error at about 14.6 and about 15.7 ppm, the peaks of about equal intensity, and (ii) a skewness index, Six, greater than about −1.20.
While the processability of these and other propylene polymers is good, a continuing interest in making it better exists, particularly with respect to shear thinning and melt strength. Increasing these attributes of processability increases the rate at which the polymer can be processed into film, fiber, extruded or injection molded articles and the like.
One method of increasing the processability of a polymer is through the use of additives, e.g., calcium stearate, a fluoro-elastomer, etc. This method, however, simply adds to the expense of the process, and these additives can build-up on the extrusion surfaces of the processing equipment.
Another method is to introduce long chain branching into the structure of the polymer. While long chain branching in polypropylene is known, it is typically the result of post-polymerization modification of the polymer chain, e.g., subjecting the polypropylene to e-beam radiation, azide coupling, etc. To introduce long chain branching into the polypropylene backbone during polymerization usually requires the use of a diene or other chain extender. Polypropylene containing long chain branching as a result of in-reactor polymerization without the use of a chain extender, e.g., a diene, is not commercially practiced.